Myc sensitizes HFFs to bortezomib-mediated, ER stress–induced apoptosis. (A) Myc activation increases intracellular calcium levels induced by BZ + SAHA. HFFs expressing the MSCV-IRES-puro or MSCV-Myc-ER^TAM-IRES-puro retroviruses were pretreated with 1 μM 4-HT for 24 hours and then treated with 100 nM BZ and 2 μM SAHA for 12 hours. Cells were collected, stained, and analyzed as described in “Measurement of intracellular Ca²⁺ levels.” Mean (n = 3), SD. * indicates a significant difference from control and ** represents a significant difference from single-agent treatments, P < .05. (B) Activation of Myc promotes caspase-4 activation after treatment with BZ + SAHA. The indicated cells were pretreated with 1 μM 4-HT for 24 hours and then treated with 100 nM BZ and 2 μM SAHA for 24 hours. Cells were collected and caspase-4 levels were detected by immunoblotting. (C,D) Caspase-4 contributes to BZ + SAHA–induced apoptosis. Caspase-4 levels were reduced using siRNA and knockdown was confirmed by immunoblotting. Cells were pretreated with 1 μM 4-HT for 24 hours and then treated with 100 nM BZ and 2 μM SAHA for 24 hours. Apoptosis was measured by PI-FACS analysis. Mean (n = 3), SD. * indicates a significant difference from nontarget siRNA–treated cells, P < .05.